Light sources are going to change in such a manner that conventional lighting devices (e.g. signal lights, lamps etc.) and systems will be more and more equipped with
LED-based light sources (LED: light emitting diodes). The main advantages of these LED light sources are their considerably higher efficiency and their increased lifetime. Because of the very small LED-dimensions and their flexible form factors, novel and interesting opportunities are offered to lamp designers.
Very often combinations of multiple LEDs (connected in series and/or in parallel) are used to increase the light output and hence to realize physically bigger, high-brightness lighting devices such as typically e.g. traffic lights. Also in the automotive area LED-based lighting devices are increasingly implemented.
Additional fields in which LED-based light sources can beneficially be employed are professional lighting applications (e.g. construction areas) as well as consumer applications (e.g. atmosphere lighting at home). Especially in these applications the requirements imposed on the light sources are high. In particular a very good color quality (e.g. color rendering) is required. Further it would be of additional interest to adapt the color itself (and/or the color temperature) of such an LED light source according to the user's demands. All this requires especially tailored electronic driving circuitries to the LED light sources.
Usually, white light is generated by means of a combination of a few different LEDs (of different colors). In principle the mix of these different colors is used to generate the desired white light (or also any other color) with the demanded characteristic. Very often red (R), green (G) and blue (B) LEDs are used. More colors such as e.g. amber (A) can be added to improve the color quality. Other color combinations can be used as well. By means of such combinations the light output (luminous flux) and the color temperature of the white light can be adjusted. Other colors can be generated by selectively combining the light generated by the available LEDs by means of electronic driving signals.
However, these possibilities require very specialized electronic driving circuits for each (or combinations) of the LEDs of such light sources. In addition, a sensor-based feedback loop (control) may be implemented in order to measure the light characteristic and to adjust the light output according to the desired properties. Conventional photosensitive devices such as e.g. light sensors (measuring the luminous flux) or (true) color sensors (measuring the light spectrum) can be employed to measure the light characteristics. Further sensors such as temperature sensing devices can be added. All sensed data will be used to feed the control circuitry of the lighting device, which will adjust the driving currents of the LEDs independently as long as the required light characteristic has been reached. The electronic driving currents of the LEDs can be modified in a large variety of basic circuits, which are state of the art. By way of example, pulse width modulation (PWM), amplitude modulation (AM) as well as direct current feeding should be mentioned.
Often, color control principles are based on color feedback loops in which color sensors are used to measure the actual light (spectrum, color). According to the sensor signal(s), the output of a solid-state based light source can be adjusted and adapted towards the required setting(s). Since the color sensors do not only recognize the radiated spectrum of the related light source (which will be controlled) but also the spectra of other interference sources (ambient light), special set-ups and/or procedures have to be used to compensate for this.
US 2006/0152725A1 discloses an LED-based measurement instrument including an illumination system and a sensing system for compensating for the contribution of ambient light. The illumination system includes modulated LEDs whereas the sensing system includes a photodiode, a transimpedance amplifier, and an integrator for the first stage to compensate for the effect of ambient light on the output side of the first stage. The components are expensive and a complex control circuit has to be implemented, which renders the device expensive.